The present invention relates to separation of maleic anhydride or phthalic anhydride from a gaseous mixture containing the anhydride, particularly by the use of an organic absorbent.
Phthalic anhydride and maleic anhydride are both important industrial chemicals which are produced by vapor-phase oxidation of a hydrocarbon feedstock in an oxidation reactor, followed by recovery and then purification of the anhydride. The most common feedstocks for phthalic anhydride plants are naphthalene and orthoxylene; for maleic anhydride plants, the most common feedstock is benzene, although other hydrocarbon feeds have been disclosed, including butene and butane.
Recovery of the anhydride from the gaseous effluent stream from the oxidation reactor can be done by scrubbing the effluent with water, which results in conversion of the anhydride to an acid. The acid then needs to be dehydrated to produce the anhydride product.
In the case of phthalic anhydride (b.p. = 285.degree. C.), condensation of the anhydride present in the reactor gaseous effluent can be done to separate the phthalic anhydride. Frequently maleic anhydride (b.p. = 202.degree. C.) is recovered by the condensation of a part of the maleic anhydride and maleic acid in the gaseous effluent, the remainder being recovered by scrubbing with water, resulting in an aqueous maleic acid solution. Then both the partial condensate and the aqueous acid solution must be dehydrated to obtain product maleic anhydride.
Recovery of maleic anhydride or phthalic anhydride from the oxidation reactor effluent using an organic absorbent as opposed to an aqueous absorbent has also been disclosed. For example, U.S. Pat. No. 2,574,644 discloses the use of dibutylphthalate for the recovery of maleic anhydride or phthalic anhydride from an oxidation reactor effluent stream. According to the process disclosed in U.S. Pat. No. 2,574,644, the oxidation reactor effluent is cooled to first condense a portion of the anhydride vapor. The remaining gaseous stream is contacted with the dibutylphthalate absorbent to remove the remaining uncondensed anhydride by absorption into the absorbent. The resulting rich absorbent is stripped to obtain a product anhydride stream.
British Pat. No. 727,828 discloses the use of dibutylphthalate for simultaneous absorption of both maleic anhydride and phthalic anhydride at absorbent temperatures above 104.degree. F.
U.S. Pat. No. 3,040,059 disclosed the use of molten wax as an absorbent for removing maleic anhydride or phthalic anhydride from an oxidation reactor effluent stream.
U.S. Pat. No. 2,893,924 discloses the use of diphenylpentachloride absorbent as well as tricresyl phosphate as an absorbent for removing maleic anhydride or phthalic anhydride by absorption.
Japanese Pat. No. 35- 7460 discloses the use of dibutylmaleate as an organic absorbent for removing maleic anhydride from gas streams, and Japanese Pat. No. 32- 8408 discloses the use of dimethylterphthalate for removing maleic anhydride from gas streams containing maleic anhydride.
Commonly assigned U.S. application Ser. No. 310,320 discloses the use of certain dialkylphthalates as absorbents for removal of maleic anhydride from gas streams containing maleic anhydride.
Also, commonly assigned U.S. application Ser. No. 209,069 discloses alkyl or alkenyl succinic anhydrides, in general intramolecular carboxylic acid anhydrides, as absorbents for the removal of maleic anhydride from gas streams. The disclosures of U.S. Ser. Nos. 310,320 and 209,069, especially in that they relate to the use of organic absorbents for maleic anhydride removal, are incorporated herein by reference.
The present invention is especially concerned with the manner of carrying out the recovery of maleic anhydride or phthalic anhydride from a hydrocarbon oxidation reactor effluent gas stream containing the anhydride in gaseous or vaporized form. Typical prior art absorption schemes involve the use of what is frequently called a "figure-8 loop". The figure is formed by the absorbent entering the top of the absorber, flowing downwardly therein, and then being transferred as rich absorbent to the absorber stripper, flowing downwardly in the stripper and then being transferred as lean absorbent to the upper part of the absorber to complete the figure-8 loop. In the absorber, the absorbent absorbs the solute and in the stripper the solute is distilled out of the absorbent. Distillation is typically accomplished by generating upflow vapors through heat input to the lower part of the stripper.
For example, the above-mentioned U.S. Pat. No. 2,574,644 shows an anhydride absorption scheme wherein basically a figure-8 loop is used. The process of this patent includes a common modification, namely the inclusion of distillation as a part of the over-all absorbent stripping operation.